Thursday, May 5, 2016

Is the Moon Waning or Waxing?

Periodically I read through one of our online course discussion forums, which has been accumulating over the past 15 years—sort of like skimming through a dictionary— and very often I run across extended notes buried on a special topic there that have long been forgotten, but deserving of some fresh air. This is an example, namely the question came up in cel nav course about deciding if the moon is waning or waxing.

So that of course first got the standard cel nav treatment, but following that were the parts that were a bit more interesting.  

Here is a copy of that forum post from 2004:

We are back from our Alaska training cruise (will write about it later), and on watch till we leave for the Pacific Cup Yacht Race on June 28. Then again off watch for 2 weeks.

Moon phases, appearances, and terminology issues are always puzzlers. There are definitely some things that change with location, but most are the same.

It is likely more an issue of which way are you looking to see the moon, rather than which hemisphere you are in at the time. At 2° N with a moon dec of 25° N, for example, i am looking north to the moon at mer pass, rather than the more common view south to mer pass from the NH.

Things that are the same:

(1) the words "waning" and "waxing" have their same English word meanings always, everywhere. Waning getting smaller, waxing getting bigger.

(2) the moon is waxing when it is moving away from the sun and waning when moving toward it. This is the key point and also one that is not so easy to picture... without a picture.

(3) the moon is full when it is precisely on the opposite side of the earth from the sun (GHA sun - GHA moon = ±180°), and there is no moon or a "new moon" when the sun and moon are on the same side (GHAs are equal).

(4) the moon moves eastward through the stars at about 12° per day (360°/30 days). This is true always, everywhere. If you see the moon next to Aldebaran bearing 190 M at 10 pm, for example on one day, then the next day at 10 pm you will see Aldebaran at roughly 191 M (sun moves 360°/365 days) and the moon will be 191-12 = 179 M. 

From Emergency Navigation

[These bearings are approximations since it is really the great circle arcs across the sky that are about 1° and 12°, not strictly their bearings on the horizon, but if their peak heights at mer pass are less than about 45°, this is a usable approximation—a point covered in some detail in the Emergency Navigation book.]

(5) the side of the moon that is lit up is the side nearest the sun. If you draw a line connecting the "two horns" of the moon — or equivalent points for other phases — and then draw a line perpendicular to that, you have a line pointing toward the location of the sun.

From Emergency Navigation

The above (5) statements are always true from any location, any time. And from these we should be able to answer all questions about moon phases.

Things that are different:

(1) When looking south to a body's daily motion, we see it's east-to-west motion as being from our left to our right, whereas looking north to view the east-to-west motion we see it moving from our right to our left.

(2) The shape of a crescent moon changes from "C" to "D," or vice versa, when looking to the north versus the south to see the moon. This is consistent with all of the above, but when it comes to making up rules on what the moon is doing (waxing- or waning-wise) based on what we see, such as the note below on whether or not the moon is "telling the truth,"  then we must qualify these with which way we are looking to see the moon, which in turn is often over simplified to specifying what hemisphere we are in.

"The Moon is always lying" the Northern Hemisphere

There is a French saying that "The moon is a liar" (La lune est menteuse), which can be used to determine if the moon is waxing or waning. I have known of this for some years but never knew what it meant, nor how to apply it. Recently a student mentioned in passing that it must mean that the shape of the moon is a letter and the letter tells us what it is doing, but it tells us wrong! 

Well, indeed, it is as simple as that. "Croître" means "to wax" and "Décroître" means "to wane," as shown in the figure. When the moon shows us a "C" (croître, waxing) it is lying, it is not waxing but waning, and vice versa when it shows us a "D" it is not décroître but croître.
Don't believe me; I am lying.

—Thanks to Capt. Jean Faubert for filling in the details on this saying, and confirming that many French-speaking navigators have indeed learned it this way.

Celtic Goddess Symbol

When I explained this discovery to my teenage daughter [actually not a teenager any longer as I re-post this today], who I thought might be interested since she studies French, she said, "Fine, but isn't it simpler to just recall the Celtic Goddess symbol?," which she immediately sketched and explained: the waxing moon on the left represents maiden; the full moon in the middle, mother; and the waning moon on the right, crone. The symbol is unforgettable, and I had to agree with her. Thanks Britt. 

Another way to check the phase in a hurry that requires no thinking, is just check out the moon page of your favorite tide app.

Still another approach...

...pointed out by our onboard instructor and skipper Capt. Steve Morrell. A letter "b" means the moon is getting bigger, ie waxing, and a letter "d" means the moon is decreasing, ie waning—and this time, nobody is lying.

And still another approach...

...pointed out in our online cel nav course by GregoryB. 

I think that the easiest way to remember waxing/waning pattern is using alliteration 

"Lit on Left is Last (waning)" 

in the northern hemisphere (opposite in southern hemisphere).


HermanD said...

Hello David<
I mentioned this before but it is never shown up here.
See the moon as one arrow with the round edge as the point of the arrow.
Now remember the standard graphic axes cross.
To the right is positive, the value grows bigger (waxing).
To th eleft is negative, the value becomes smaller (waning)

Moon-arrow to right waxing.
Moon-arrow to left waning.
Is n't that the simplest to remember most logical waxing/waning rule?
Kind Regards,

David Burch said...

Thanks Herman. Good idea. We have the arrows and conclusions in the first figure above, but your idea to think of positive axis (waxing) to the right is a nice touch. Thanks again.